uncertainty -
The standarddeviation of a sufficiently large number of measurements of the same quantity by the same instrument or method. Hence, the noncorrectable part of the inaccuracy of an instrument; it represents the limit of measurement precision. The uncertainty of an instrument is caused by the unpredictable effects upon its performance of such factors as friction, backlash, and electronic noise.

uniform flow -
Flow in open channels or closed conduits where flow characteristics such as depth or mean velocity do not change with distance.

unstable air -
Air in which static instability prevails. This condition is determined by the vertical gradients of air temperature and humidity.

upper-air -
(Also aerological.) Having to do with the free atmosphere, including the troposphere and stratosphere. Upper-air observations are distinguished from surface observations, even though an upper-airobservation may include data from the surface.

upper-level cyclone -
(Also called upper-level low, upper cyclone, upper low, high-level cyclone, low aloft.) A cyclonic circulation existing in the upper air; specifically as seen on an upper-level constant-pressure chart. This term is often restricted to such cyclones associated with relatively little cyclonic circulation in the lower atmosphere.

upper-level low -
Same as upper-level cyclone.

upper-level ridge -
(Also called upper ridge, upper-air ridge, high-level ridge, ridge aloft.) A pressure ridge existing in the upper air, especially one that is stronger aloft than near the earth's surface.

upper-level trough -
(Also called upper trough, upper-air trough, high-level trough, trough aloft.) A pressuretrough existing in the upper air. This term is sometimes restricted to those troughs that are much more pronounced aloft than near the earth's surface. These troughs are often described as either short-wave or long-wave features.

upper-level winds -
Same as winds aloft.

upper air -
In synopticmeteorology and in weather observing, that portion of the atmosphere that is above the lower troposphere. No distinct lower limit is set but the term can be generally applied to the levels above 850 mb. Compare upper atmosphere.

upper atmosphere -
The general term applied to the atmosphere above the troposphere. See atmospheric shell.